JP5099107B2 - Electrolytes and electrochemical devices - Google Patents

Description

The present invention relates to an electrolyte and an electrochemical device using an ionic compound.

In recent years, there has been an increasing demand for reduction in size and weight of various devices using batteries as a power source, such as portable devices, and there is a strong demand for batteries having further improved battery characteristics. Therefore, high performance is also demanded for the electrolyte that is one component of the battery.
Conventionally, inorganic salt compounds such as LiBF ₄ and LiPF ₆ have been widely known as electrolyte salts used for lithium batteries, for example. Since the inorganic salt compounds listed above do not have fluidity as a simple substance, in order to obtain sufficient ionic conductivity for use as an electrolyte for lithium batteries, the inorganic salt compound is usually an organic compound such as propylene carbonate or ethylene carbonate. An electrolytic solution dissolved in a solvent is used.

  However, when the above electrolyte is used, the cation (lithium ion) has a mobility smaller than that of the anion, so that the lithium ion transport number is insufficient, which is one factor that limits the electric characteristics of the lithium battery. It was. For this reason, when such an electrolyte is used as an electrolyte for an electrochemical device that is used for applications having a large DC component, such as a battery, the salt concentration on the anode side (the negative electrode side during discharging or the positive electrode side during charging) increases. In addition, there is a problem that the polarization of the electrochemical device is increased because the salt concentration cannot be sufficiently relaxed by the diffusion effect.

  Therefore, by using a zwitter ion (Zwitter ion) compound with a specific structure, which is considered to have low mobility under a potential gradient, and a lithium salt, the lithium ion transport number is increased by suppressing the movement of the anion. The technique to try is known. (See Non-Patent Document 1)

Yoshizawa, M., Hirao, M., Ito-Akita, K., Ohno, H., “Journal of Materials Chemistry ) ", (UK), 2001, Vol. 11, p. 1057

  However, in the above-described prior art, a lithium salt must be added as a source of lithium ions as carrier ions. However, since the counter anion of the lithium salt can freely move, the lithium ion transport number is still not sufficient.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an electrolyte containing an ionic compound having a high carrier ion transport number and an electrochemical device excellent in electrical characteristics. .

In order to achieve the above object, the ionic compound contained in the electrolyte according to the present invention (hereinafter referred to as “ionic compound according to the present invention”) has a molecular structure portion having a positive charge in the same ion and a negative charge. And an organic ion in which either positive or negative charge is excessively present and a carrier ion having a charge opposite to the overall charge of the organic ion.
According to such a configuration, the direction in which the molecular structure portion having the positive charge of the organic ion moves in the potential gradient is opposite to the direction in which the molecular structure portion having the negative charge of the organic ion moves. As a result, the mobility of organic ions is extremely small. Furthermore, there is no need to add an ionic compound capable of supplying carrier ions, and there is no “carrier ion counter ion” that can freely move, so that the carrier ion transport number can be further improved.
Here, the molecular structure portion having the positive charge, the molecular structure portion having the negative charge, and the molecular structure portion having the negative charge so that the molecular structure portion having the positive charge and the molecular structure portion having the negative charge are not different ions. Are preferably bonded at least in one place by a bond that does not ionically dissociate. Thereby, the organic ion which expresses the said function can be made to exist stably.
In addition, since the charge of the organic ions as a whole is negative, the cation can be used as a carrier ion, and the cation transport number such as an alkali metal ion can be improved.

［式中、Ｒ₁、Ｒ₂は、それぞれ独立して、水素原子またはメチル基を示す。Ｍ^＋はアルカリ金属イオンを示す。ｎは３以上１８以下の整数である。］ More specifically, the ionic compound according to the present invention is preferably represented by the following general formula (I).

[Wherein, R ₁ and R ₂ each independently represent a hydrogen atom or a methyl group. M ⁺ represents an alkali metal ion. n is an integer of 3 to 18. ]

［式中、Ｒ₁、Ｒ₂は、それぞれ独立して、水素原子またはメチル基を示す。Ｍ^＋はアルカリ金属イオンを示す。ｉ、ｊは、それぞれ独立して、３以上１８以下の整数である（ただし、ｉ＝ｊである場合は除く）。］ More specifically, the ionic compound according to the present invention is preferably represented by the following general formula (IV).

[Wherein, R ₁ and R ₂ each independently represent a hydrogen atom or a methyl group. M ⁺ represents an alkali metal ion. i and j are each independently an integer of 3 or more and 18 or less (except when i = j). ]

The present inventors give an ionic compound having a low melting point, and a quaternary ammonium cation structure having a tetravalent nitrogen atom, as seen in an imidazolium-based ionic compound, and in some cases becomes a room temperature molten salt. attention to, which was adopted in the ionic compound of the present invention to.
According to the ionic compound according to the present invention described above, the anion has an imidazole ring and is bulky compared to the alkali metal ion that is a cation. The degree can be reduced.
Furthermore, the anion of the ionic compound according to the present invention is a triple ion type anion having two sulfonate anions and one imidazolium cation, and the sulfonate anion tends to move under a potential gradient. Since the direction and the direction in which the imidazolium cation tends to move are opposite, as a result, the mobility of the triple ion type anion becomes extremely small.
Furthermore, it is not necessary to add an alkali metal salt as an alkali metal ion source, and there is no counter anion that can freely move, so that the alkali metal ion transport number can be further improved.
Therefore, according to the ionic compound which concerns on this invention , it can be set as an ionic compound with a high alkali metal ion transport number.

Here, it is preferable that at least one of R ₁ and R ₂ is a methyl group, and in particular, the ion conductivity can be made excellent.

Therefore, since the electrolyte according to the present invention contains the above-described ionic compound according to the present invention, the electrolyte can have a high alkali metal ion transport number.

  Since the electrochemical device according to the present invention includes the electrolyte according to the present invention, the electrochemical device having excellent electrical characteristics can be obtained.

According to the electrolyte of the present invention, since the ionic compound having a high carrier ion transport number is contained, an electrolyte having a high carrier ion transport number can be provided.

  According to the electrochemical device according to the present invention, since the electrolyte according to the present invention is provided, an electrochemical device having excellent electrical characteristics can be provided.

  Although the embodiment of the present invention is illustrated below, the present invention is not limited to the following embodiment.

  An ionic compound according to an embodiment of the present invention has a molecular structure part having a positive charge and a molecular structure part having a negative charge in the same ion, and an organic ion in which either positive or negative charge is excessively present. And carrier ions having a charge opposite to that of the organic ions as a whole. Here, it is preferable that the molecular structure part having the positive charge and the molecular structure part having the negative charge of the organic ion are bonded at least in one place by a bond that is not ion-dissociated. Moreover, the form which the electric charge as a whole of organic ion becomes negative can be illustrated suitably.

［式中、Ｒ₁、Ｒ₂は、それぞれ独立して、水素原子またはメチル基を示す。Ｍ^＋はアルカリ金属イオンを示す。ｎは３以上１８以下の整数である。］ More specifically, the ionic compound according to the embodiment of the present invention is preferably represented by the following general formula (I).

[Wherein, R ₁ and R ₂ each independently represent a hydrogen atom or a methyl group. M ⁺ represents an alkali metal ion. n is an integer of 3 to 18. ]

［式中、Ｒ₁、Ｒ₂は、それぞれ独立して、水素原子またはメチル基を示す。Ｍ^＋はアルカリ金属イオンを示す。ｉ、ｊは、それぞれ独立して、３以上１８以下の整数である（ただし、ｉ＝ｊである場合は除く）。］ Moreover, it is preferable that the ionic compound which concerns on embodiment of this invention is represented by the following general formula (IV).

[Wherein, R ₁ and R ₂ each independently represent a hydrogen atom or a methyl group. M ⁺ represents an alkali metal ion. i and j are each independently an integer of 3 or more and 18 or less (except when i = j). ]

In particular, it is preferable that at least one of R ₁ and R ₂ is a methyl group, which can improve ion conductivity. In addition, since an ionic compound in which at least one of R ₁ and R ₂ is a methyl group has a particularly wide potential window on the reduction side, an electrolyte containing these ionic compounds is a high electromotive force electrochemical device. Can be applied to.
In particular, when R ₁ is a methyl group and R ₂ is a hydrogen atom, the case where R ₁ is a hydrogen atom and R ₂ is a methyl group is preferable from the viewpoint of developing high ionic conductivity.
In the chemical formula (I), n is preferably 3 or more and 18 or less, or in the chemical formula (IV), i and j are each preferably 3 or more and 18 or less. In addition, when n, i, j is less than 3, the molecular structure portion having a positive charge and the molecular structure portion having a negative charge are too close to each other. This is not preferable because the melting point and the glass transition temperature of the glass are increased. On the other hand, if n, i, j exceeds 18, the molecular weight of the ionic compound becomes too large, so that the melting point and glass transition temperature of the ionic compound rise, which is not preferable.

Particularly preferable examples of the alkali metal ion include lithium ions (Li ⁺ ), sodium ions (Na ⁺ ), potassium ions (K ⁺ ), and cesium ions (Cs ⁺ ).

  Below, the preferable specific example of an ionic compound represented with general formula (I) is illustrated.

  Below, the suitable manufacturing method of the ionic compound represented by general formula (I) is demonstrated.

[When M ⁺ is lithium ion]
First, an imidazole compound represented by the following general formula (II) (R ₁ and R ₂ are the same as those in the general formula (I), the same applies hereinafter) and lithium hydride (LiH) are mixed with an inert gas such as argon. The mixture is mixed in an aprotic solvent such as N, N-dimethylacetamide at room temperature (0 ° C. to 30 ° C.) for 1 hour to 2 hours, and then this solution is represented by the general formula (III) A sultone compound (n is the same as in the general formula (I); the same applies hereinafter) is added and mixed for 10 hours or more. Here, it is preferable that the imidazole compound represented by the general formula (II), the lithium hydride, and the sultone compound represented by the general formula (III) are approximately equimolar amounts. The product is washed several times with an organic solvent such as acetone and dried at a temperature between 60 ° C. and 80 ° C. under reduced pressure. The obtained substance is dissolved in an aprotic solvent such as dimethyl sulfoxide, and 130% by weight to 140% by weight of the sultone compound represented by the general formula (III) is added to the obtained substance. Mix at room temperature (0 ° C. to 30 ° C.) for 48 hours to 96 hours. The product is washed several times with an organic solvent such as ethanol and dried under reduced pressure at a temperature of 60 ° C. to 80 ° C. to obtain an ionic compound (M ⁺ = lithium ion) represented by the general formula (I). It is done. In addition, as needed, the deposit deposited by dripping the solution after reaction to poor solvents, such as acetone, can also be wash | cleaned with the organic solvent as said product.

Examples of the imidazole compound represented by the general formula (II) include imidazole, 2-methylimidazole, 4-methylimidazole, and 2,4-dimethylimidazole, and commercial products such as those manufactured by Tokyo Chemical Industry Co., Ltd. are available. is there.
Examples of the sultone compound represented by the general formula (III) include 1,3-propane sultone and 1,4-butane sultone, and commercially available products such as those manufactured by Tokyo Chemical Industry Co., Ltd. and Kanto Chemical Co., Inc. are available. .

[When M ⁺ is an alkali metal ion other than lithium ion]
An imidazole compound represented by the general formula (II) and an alkali metal ethoxide represented by the chemical formula CH ₃ CH ₂ OM (M is an alkali metal other than lithium) are mixed at room temperature (0 ° C. to 0 ° C.) under an inert gas atmosphere such as nitrogen. At 30 ° C.) in a polar solvent such as ethanol for 1 to 2 hours, and then sultone compound represented by general formula (III) (n is the same as in general formula (I). )) And mix for at least 10 hours. Here, it is preferable that the imidazole compound represented by the general formula (II) and the alkali metal ethoxide are approximately equimolar amounts, and the sultone compound represented by the general formula (III) is represented by the general formula (II). It is preferable that the molar amount is at least twice that of the imidazole compound represented. The product is washed several times with an organic solvent such as ethanol and dried at a temperature of 60 to 80 ° C. under reduced pressure, whereby an ionic compound represented by the general formula (I) (M ⁺ = alkali metal other than lithium ion) Ion) is obtained. In addition, as needed, the deposit deposited by dripping the solution after reaction to poor solvents, such as acetone, can also be wash | cleaned with the organic solvent as said product.

Examples of the alkali metal ethoxide include sodium ethoxide and potassium ethoxide, and commercially available products such as those manufactured by Aldrich are available.
Examples of the imidazole compound represented by the general formula (II) and the sultone compound represented by the general formula (III) include those listed above.

The ionic compound represented by the general formula (IV) is, for example, in a suitable method for producing the ionic compound represented by the general formula (I) (see the above [when M ⁺ is a lithium ion]). In the first reaction between the imidazole compound and the sultone compound, a sultone compound in which n of the sultone compound represented by the general formula (III) is changed to i is used, and in the second reaction between the imidazole compound and the sultone compound, It can be produced by using a sultone compound in which n of the sultone compound represented by the general formula (III) is changed to j.

According to the ionic compound which concerns on embodiment of this invention demonstrated above, it can be set as an ionic compound with a high alkali metal ion transport number.
As an application example of the ionic compound, an electrolyte such as an electrochemical device can be preferably mentioned as described later.

Therefore, the electrolyte according to the embodiment of the present invention contains the ionic compound according to the above-described embodiment of the present invention, and can increase the alkali metal ion transport number.

Since the electrochemical device according to the embodiment of the present invention includes the electrolyte according to the embodiment of the present invention, an electrochemical device having excellent electrical characteristics can be obtained. Examples of the electrochemical device include a lithium primary battery, a lithium secondary battery, a lithium ion battery, a fuel cell, and an electric double layer capacitor.
When the electrochemical device according to the embodiment of the present invention is a lithium battery, since the ionic compound preferably has lithium ions as carrier ions, the general formula (I) in which M ⁺ is a lithium ion or The compound represented by the formula (IV) is preferably contained.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, these do not limit this invention.

Example 1
Imidazole (3.01 g, 0.044 mol) and lithium hydride (0.35 g, 0.044 mol) in an N, N-dimethylacetamide (60 ml) at 20 to 30 ° C. under an argon atmosphere for 10 hours. Mixed. Next, 1,3-propane sultone (5.37 g, 0.044 mol) was added to this solution and mixed for 10 hours. The product was washed 3 times with acetone and dried at 60 ° C. under reduced pressure. The obtained white powder (1.03 g) was dissolved in dimethyl sulfoxide, and 1,3-propane sultone (0.76 g) was added to this solution, and mixed at 20 ° C. to 30 ° C. for 4 days. The product was washed twice with ethanol and dried at 60 ° C. under reduced pressure to obtain a compound represented by the above chemical formula (1-1) (see ¹ H-NMR measurement result below). It was 287 degreeC when melting | fusing point of the compound represented by Chemical formula (1-1) was measured. The compound represented by the chemical formula (1-1) is used as the electrolyte of Example 1.

¹ H-NMR (500 MHz, solvent: D ₂ O) chemical shift (ppm): 2.30 to 2.36 (m, 4H), 2.94 (t, J = 7 Hz, 4H), 4.38 (t , J = 7 Hz, 4H), 7.57 (s, 2H), 8.88 (s, 1H)

(Example 2)
Imidazole (4.29 g, 0.063 mol) and sodium ethoxide (4.29 g, 0.063 mol) were mixed in ethanol (80 ml) at 20 ° C. to 30 ° C. for 4 hours under a nitrogen atmosphere. Next, 1,3-propane sultone (19.2 g, 0.158 mol) was added to this solution and mixed at 20 ° C. to 30 ° C. for 2 hours. The product was washed twice with ethanol and dried at a temperature of 60 ° C. under reduced pressure to obtain a compound represented by the above chemical formula (1-2) (see ¹ H-NMR measurement result below). The compound represented by the chemical formula (1-2) is used as the electrolyte of Example 2. It was 254 degreeC when melting | fusing point of the compound represented by Chemical formula (1-2) was measured.

¹ H-NMR (500 MHz, solvent: D ₂ O) chemical shift (ppm): 2.30 to 2.36 (m, 4H), 2.94 (t, J = 7 Hz, 4H), 4.38 (t , J = 7 Hz, 4H), 7.57 (s, 2H), 8.88 (s, 1H)

(Example 3)
2-Methylimidazole (2.40 g, 0.029 mol) and sodium ethoxide (1.97 g, 0.029 mol) were mixed in ethanol (80 ml) at 20 ° C. to 30 ° C. for 2 hours under a nitrogen atmosphere. did. Next, 1,3-propane sultone (8.79 g, 0.072 mol) was added to this solution and mixed at 20 ° C. to 30 ° C. for 2 hours. The product was washed twice with ethanol and dried at 60 ° C. under reduced pressure to obtain a compound represented by the above chemical formula (1-3) (see ¹ H-NMR measurement result below). The compound represented by the chemical formula (1-3) is used as the electrolyte of Example 3.

¹ H-NMR (500 MHz, solvent: D ₂ O) chemical shift (ppm): 2.33 to 2.39 (m, 4H), 2.75 (s, 3H), 3.06 (t, J = 7 Hz) , 4H), 4.39 (t, J = 7 Hz, 4H), 7.56 (s, 2H)

Example 4
4-Methylimidazole (2.40 g, 0.029 mol) and sodium ethoxide (1.97 g, 0.029 mol) were mixed in ethanol (80 ml) at 20 ° C. to 30 ° C. for 3 hours under a nitrogen atmosphere. did. Then, 1,3-propane sultone (10.6 g, 0.087 mol) was added to this solution and mixed at 20-30 ° C. for 1 day. This solution was added dropwise to 20 times the amount of acetone with respect to the solution, and the deposited precipitate was washed twice with ethanol and dried at a temperature of 60 ° C. under reduced pressure, whereby the chemical formula (1-4) The compound represented was obtained (see ¹ H-NMR measurement results below). The compound represented by the chemical formula (1-4) is used as the electrolyte of Example 4.

¹ H-NMR (500 MHz, solvent: D ₂ O) chemical shift (ppm): 2.23 to 2.36 (m, 4H), 2.34 (s, 3H), 2.92 (t, J = 7 Hz) , 2H), 2.98 (t, J = 7 Hz, 2H), 4.32 (m, 4H), 7.32 (s, 1H), 8.76 (s, 1H)

(Comparative Example 1)
A solution containing 10% by weight of LiCF ₃ SO ₃ (solvent: propylene carbonate-methylpropionate mixed solvent) is used as the electrolyte of Comparative Example 1.

(Comparative Example 2)
N-ethylimidazole and methanesulfonic acid were slowly mixed in ethanol under ice-cooling and stirred for 1 day for reaction. Ethanol was removed by evaporation, washed with diethyl ether, and dried at a temperature of 60 ° C. under reduced pressure to obtain a compound represented by the following chemical formula (A). The compound represented by the chemical formula (A) and LiN (CF ₃ SO ₂ ) ₂ are mixed in equimolar amounts to obtain the electrolyte of Comparative Example 2.

(Comparative Example 3)
N-ethylimidazole and propane sultone were slowly mixed in acetone under ice-cooling and nitrogen and stirred for 5 days to be reacted. The precipitate was washed with acetone, diethyl ether or the like, and dried at a temperature of 60 ° C. under reduced pressure to obtain a compound represented by the following chemical formula (B). The compound represented by the chemical formula (B) and LiN (CF ₃ SO ₂ ) ₂ are mixed in equimolar amounts to obtain the electrolyte of Comparative Example 3.

(Measurement of alkali metal ion transport number)
Next, the electrical characteristics of the electrolyte according to the example were evaluated. Using the electrolytes of Examples and Comparative Examples, an electrochemical device having a pair of carbon electrodes storing lithium ions as a non-blocking electrode was produced. The lithium ion transport number was measured by the direct current polarization method. The measurement temperatures were all 300 ° C. The results are shown in Table 1.

As shown in the results of Table 1, the electrolyte of the example has an improved lithium ion transport number compared to the electrolyte of the comparative example, and has a sufficient lithium ion transport number as an electrolyte for an electrochemical device. It was confirmed that At the same time, it was proved by the measurement that the electrochemical device using the same has suppressed polarization.
From the above, when the electrolyte according to the present invention is used as an electrolyte of electrochemical devices such as lithium primary batteries, lithium secondary batteries, lithium ion batteries, fuel cells, electric double layer capacitors, etc. Since polarization when used can be reduced, it is possible to provide a high electrochemical device excellent in electrical characteristics such as discharge performance and repeated charge / discharge cycle performance.

In the ionic compound according to the present invention, by taking the following measures, the melting point or glass transition point can be made lower than in the examples, and the ionic conductivity is maintained while maintaining a high carrier ion transport number. It can be made even better.
First, by increasing the distance between the molecular structure portion having a positive charge and the molecular structure portion having a negative charge, an ionic compound having a better ionic conductivity can be obtained. That is, the value of n of the ionic compound in the general formula (I) and the values of i and j of the ionic compound in the general formula (IV) are set to 4 or more (n, i, j = 4, 5, etc.). Thus, since crystallization is suppressed, the melting point of the ionic compound can be lowered, and the ionic conductivity can be further improved.
Second, by selecting carrier ions having a large ionic radius, an ionic compound with even better ionic conductivity can be obtained. As can be seen from the measurement results of the melting points of the compound represented by the chemical formula (1-1) and the compound represented by the chemical formula (1-2), it is represented by the general formula (I) in which M ⁺ is a sodium ion. The ionic compound to be produced has a lower melting point than the ionic compound represented by the general formula (I) in which M ⁺ is a lithium ion. That is, since the alkali metal ion radius is Li <Na <K <Cs, the melting point of the ionic compound can be lowered by making M ⁺ an alkali metal ion other than lithium, and the ionic conductivity is further improved. Can be.
The present inventors give an ionic compound having a low melting point, and a quaternary ammonium cation structure having a tetravalent nitrogen atom, as seen in an imidazolium-based ionic compound, and in some cases becomes a room temperature molten salt. This was adopted as an ionic compound comprising the triple ion type anion of the present invention. Therefore, the above-mentioned measures and other approaches that make the melting point or glass transition point lower than those of the examples can provide an ionic compound composed of a triple ion type anion that exhibits a liquid at room temperature.

Note that the melting point of the ionic compound or the molecular structure portion having a positive charge and the molecular structure portion having a negative charge, or the structure of the molecular structure portion having a positive charge or a negative charge itself may also be affected. In some cases, the glass transition point may be lower than in the examples. For example, the glass transition point of the ionic compound can be greatly reduced by introducing a polar group into the structure of the alkyl group bonded to the nitrogen atom of the ionic compound in general formula (I). Here, as a polar group, an ether group etc. are mentioned, for example. That is, by making at least a part of the alkyl group an ether structure represented by, for example, — (CH ₂ CH ₂ O) _n —, the glass transition point of the ionic compound can be greatly reduced. Further, the glass transition point of the ionic compound may be lowered by introducing a polar group into the group represented by R ₁ or R ₂ of the ionic compound represented by the general formula (I). In this case, examples of the polar group include an ether group and a cyano group.

Claims (3)

In the same ion and a molecular structure portion having one imidazolium cation structure portion and two negative charges, the molecular structure portion having two negative charges, each of the two nitrogen atoms of the imidazolium cationite down Each containing an ionic compound which is bound by a non-ionically dissociating bond, wherein the ion as a whole has a negative monovalent organic ion and one alkali metal ion forms an ion pair , you an alkali metal ion as a carrier ion, electrolyte.   The molecular structure portion having a negative charge includes a sulfonate anion as the negative charge. 1. The electrolyte according to 1. Electrochemical device comprising an electrolyte according to claim 1 or 2.